There is a power inverter that is mounted on an electrical-powered vehicle such as an electrical vehicle (EV) or a hybrid electrical vehicle (HEV) and is connected to a driving electrical motor. The power inverter has a power conversion function of converting DC power supplied from a DC power source and used to drive an electrical motor into AC power or converting AC power generated from the electrical motor into DC power for charging the DC power source. To realize the power conversion function, a switching element such as a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor) is generally used in the power inverter. When a voltage greater than a predetermined resistance voltage is applied, resistance voltage breakdown may occur and thus the switching element may be damaged. Accordingly, the applied voltage is required not to be greater than the resistance voltage.
In a general electrical-powered vehicle, in order to efficiently use power, an electrical motor is forcibly rotated at the time of braking or at the time of downhill driving so as to work as a generator and a regenerative voltage (back-electromotive force) is generated to charge a DC power source. However, a higher regenerative voltage is generated as the number of times that the electrical motor is rotated increases. Therefore, when the regenerative voltage is greater than the resistance voltage, the resistance voltage breakdown may occur in a switching element. Accordingly, to prevent the resistance voltage breakdown, means for suppressing an overvoltage is provided in a conventional power inverter.
For example, PTL 1 suggests a method of suppressing an overvoltage in the following way in an electrical motor control device controlling an electrical motor. In this method, by turning on a switching element depending on a rotation position of the electrical motor under the control of a CPU when an overvoltage occurs, a plurality of phases of the electrical motor is connected in series.
Further, PTL 2 describes the following method. In this method, a regenerative voltage input from an inverter to a DC power source is detected. Inverter control means controls a switching element of the inverter so that opening or short-circuiting between lines of an electrical motor driving device and an electrical motor is performed by at least one of the electrical motor driving device and the electrical motor based on the detection result.